1. Field of the Invention
The present invention relates to a vehicle occupant protection apparatus for protecting a vehicle occupant by controlling an operation of an occupant protection device (e.g. an airbag apparatus) according to a type of collision while a vehicle is colliding, and an initiation method to use for the vehicle occupant protection apparatus.
2. Description of Related Art
Conventionally, there is known a vehicle occupant protection apparatus for determining greatness of collision velocity by using a change in acceleration with time (a time-integrated value of acceleration and a time-differentiated value of acceleration) detected by a single acceleration sensor (for reference, see Japanese Patent Laid-Open No. H10-29494).
In the above apparatus, each of two inflators is not driven when the collision velocity is notably small, and only a first inflator is driven to half unfold an airbag (to inflate an airbag with a low pressure) when the collision velocity is medium, and further, after the first inflator is driven a second inflator is driven to fully unfold the airbag (to inflate the airbag with a high pressure) when the collision velocity is great.
Also, there is known a vehicle occupant protection apparatus which detects deceleration with a single acceleration sensor, then calculates a first integrated value which is a time integrated value of the deceleration, then calculates a second integrated value by taking the time-integral of the first integrated value, then adds a weight value to each of the first integrated value and the second integrated value, and then drives an airbag by estimating movement of a vehicle occupant using the calculated sum (for reference, see Japanese Patent Publication No. H08-25430)
It is proposed that a vehicle occupant protection apparatus detects acceleration with a single acceleration sensor thereof, then calculates acceleration, greatness of the acceleration, change in acceleration with time and a reduced quantity of velocity with a control circuit thereof, then estimates a scale of an impact by using those calculated values, and then drives each inflator according to the estimated scale (for reference, see Japanese Patent Laid-Open No. 2004-131084).
It is also proposed that an initiation control apparatus for use on a vehicle occupant protection apparatus detects deceleration of a vehicle with a single acceleration sensor thereof, then calculates a integral velocity value, which is a time integrated value of the deceleration, then detects when the integral velocity value exceeds a first predetermined threshold value and initiates a first inflator, and then initiates a second inflator at the time the integral velocity exceeds a second predetermined threshold value, so that the second inflator can be driven without fault in the case the time integrated value of the deceleration delays in responding after the first inflator is driven in an offset collision, by improving arithmetic calculation process (for reference, see Japanese Patent Laid-Open No. H11-263188).
Normally, when protecting a vehicle occupant from a collision impact by way of unfolding an airbag, since the time for inflating an airbag should be considered, a type of the collision needs to be determined until 30 milliseconds (0.03 seconds) before the impact of the collision which may damage the occupant applies to the occupant to inflate the airbag according to the type of the collision.
That is, as described in H11-263188, there is a case where an airbag is fully unfolded (is inflated with a high pressure), and there is another case where the airbag is half unfolded (is inflated with a low pressure), according to the type of a collision.
However, in the vehicle occupant protection apparatus described in H11-263188, if the first and second threshold values are predetermined in order to drive the first inflator when a low velocity head-on collision occurred and to drive the first and second inflator when a high velocity head-on collision occurred, then there may rise a case where the integrated value of the acceleration exceeds the first threshold value but does not exceed the second threshold value when a high velocity oblique collision or a high velocity offset collision occurs, and in such a case the second inflator may not be driven.
FIG. 5 is a graph showing the problem described above. The lateral axis designates elapsed time from the beginning of a collision occurs, and the vertical axis designates integrated values of acceleration (time integrated values from the time collision occurs) detected with an acceleration sensor. The acceleration sensor may be attached either to an anterior-end portion of a vehicle or to a substantially central portion of the vehicle. In the graph “th1” designates a threshold value for initiating the first inflator and “th2” designates a threshold value for initiating the second inflator. The symbol “a” designates a characteristic of change in the integrated value in the case where a head-on collision occurs at a low velocity (e.g. 26 kilometer/hour). The symbol “b” designates a characteristic of change in the integrated value in the case where a head-on collision occurs at a high velocity (e.g. 35 kilometer/hour). The symbol “c” designates a characteristic of change in the integrated value in the case where an offset/oblique collision occurs at a high velocity (e.g. an offset collision at 64 kilometer/hour or an oblique collision at 40 kilometer/hour). As indicated with the symbol “c”, in a high velocity offset collision or a high velocity oblique collision there might occur the case where the integrated value of the acceleration exceeds the first threshold value th1 but does not exceed the second threshold value th2, and in such a case the second inflator may not be driven. Further, in the case where a high velocity offset collision or a high velocity oblique collision occurs, it might take time for the integrated value of acceleration to reach the second threshold value, thus the timing for initiating the second inflator might not be proper. Further to this, another vehicle occupant protection apparatus is described in Japanese Patent Laid-Open No. 2001-10441.